Most modern aircraft have nose landing gear situated under the nose of the fuselage of the aircraft. Generally, steering of the aircraft on the ground is controlled by controlling the steering of a steerable bottom portion of the nose landing gear that carries the wheels, with steering orders being generated for a steering actuator, e.g. an actuator with a rack or a rotary actuator co-operating with a toothed ring constrained to turn with the steerable bottom portion, or indeed a push-pull device.
The angular position of the steerable portion of the auxiliary landing gear is servo-controlled to a command order coming from the cockpit. This servo-control is performed in conventional manner by using a proportional integral derivative (PID) type controller which outputs a command current for delivery to the steering actuator.
The command order is generated by a pilot or a copilot operating a steering wheel specially provided for this purpose, or pedals, or both of those elements simultaneously. Sensors for sensing the angular position of the nose landing gear measure the angular position of the steerable portion in order to servo-control the command order. Those sensors are generally rotary variable differential transformer (RVDT) sensors, or potentiometer type sensors.
The use of such angle sensors for determining the steering angle of the aircraft makes steering control liable to measurement errors, e.g. as a result of tires that are not properly inflated, as a result of the landing gear being out of balance, or as a result of variations in the positioning of the sensors.
In addition, such angle sensors are situated on the nose landing gear in a zone that is particularly exposed, being subjected in particular to high levels of mechanical stress, to large temperature variations, and to high levels of moisture. The reliability of such sensors is degraded as a result of their position, which means they need to be replaced frequently.